To determine volume and mass flows in flowing gases, it is usual and meaningful to determine flow parameters at individual measuring points and to determine the overall flow from these measured values by extrapolation over time and/or space. Flow parameters are either measured for this purpose directly or derived from the measurement of other physical variables, which unambiguously depend on the flow parameters, by the use of various evaluation algorithms. The measurement method is selected, as a rule, depending on a great variety of requirements and boundary conditions. These include the space available, the necessary accuracy, the costs associated with the measurement, the presence of supply media, various requirements on handling and comfort and the applicability of suitable measuring means.
The fields of applications in which the boundary conditions must be taken into consideration include various flow measurements in connection with the performance of the artificial respiration of patients. The present invention will therefore be explained below largely in respect to medical applications, without wishing to explicitly limit the present application to this field of application.
To make it possible to obtain reliable information on the respiration status of a patient from flow measurements, distortions due to a possible elastic deformation of the pipeline system for guiding the gas must be avoided. Measuring points located as close to the patient as possible are therefore usually desirable. Furthermore, the measuring set-up is to be designed such that the flow resistance will be low in order to keep the load on the patient as low as possible. Design-related increases in the dead space are to be avoided especially in case the respiratory system is compromised or in case of small tidal volumes. Due to use near the patient in the field of clinical medicine, there must be, moreover, insensitivity to particles being carried within the gas flow and to sputum. Since flow balances are frequently to be established from the flow measurements, reliable information on the direction of flow is, moreover, usually desirable.
It is known that flow measurements can be carried out in the respiration system by the use of so-called hot wire anemometers (DE 10104462 A1). Hot wire anemometers are usually accommodated for this purpose in separate components, which ensure that the flow to be measured will become uniform, and possibly near the patient, i.e., usually between the Y-piece and the tube or directly at or after the Y-piece. Due to accommodation in a separate component, the dead space will become larger or arrangement at a relatively great distance from the patient will become necessary.
Another drawback of this process is its sensitivity to particles being carried in gas flows as well as contamination by sputum. Since it is an electric measurement method, supply lines to the measuring element proper are, moreover, always necessary, which may lead to disadvantages in clinical practice in terms of applicability and user friendliness.
DE 103 52 652.8 relates to carrying out flow measurements in respiration systems by ultrasound measurements. A defined measuring section, which is investigated with ultrasound, is usually preset for this by a separate component. By evaluating the received ultrasound, it is possible to obtain information on the velocity of flow and the direction of flow. One drawback of this process is likewise the usually inevitable enlargement of the dead space as well as the complicated electronic evaluating and supply system, which must be connected directly to the measuring element in this case as well.